A two-wavelength infrared photodetector, which is a photodetector, may detect infrared of two different wavelengths. Such a two-wavelength infrared photodetector has pixels that convert infrared signals in two different wavelength regions into electrical signals, is connected to, for example, a drive circuit, and is used, for example, as an infrared image sensor.
In the two-wavelength infrared photodetector, absorber layers that respond to infrared of two different wavelengths are stacked with a contact layer interposed therebetween. The contact layer is connected to the drive circuit. Examples of the photodetector include a quantum well infrared photodetector (QWIP) and a quantum dot infrared photodetector (QDIP). Each absorber layer of the QWIP is formed of multi-quantum well layers. Each absorber layer of the QDIP is formed of quantum dots layers. Carriers bound at a quantum level in a quantum well in the case of the QWIP or in a quantum dot in the case of the QDIP are detected as optical signals in a manner in which the carriers absorb infrared and are extracted from the contact layer.
In recent years, since there has been a demand for an increased number of pixels and improved accuracy in two-wavelength infrared photodetectors, the area of each pixel has decreased. For example, a structure in which a single bump electrode is formed on the pixels is disclosed.
A two-wavelength infrared photodetector including a single bump electrode will be described with reference to FIG. 1. In the two-wavelength infrared photodetector, a lower contact layer 921, a first absorber layer 931, an middle contact layer 922, a second absorber layer 932, an upper contact layer 923, and a coupler layer 924 are formed of compound semiconductors so as to be stacked on a GaAs substrate 910. The lower contact layer 921, the middle contact layer 922, and the upper contact layer 923 are formed of n-GaAs. A diffraction grating is formed on a surface of the coupler layer 924, so that an optical coupling structure is formed. The first absorber layer 931 and the second absorber layer 932 each have multi-quantum well (MQW) structures. The first absorber layer 931 and the second absorber layer 932 are formed so as to be able to detect infrared of different wavelengths.
Pixel-separating grooves 902 are formed to separate pixels 901 from each other. The pixel-separating grooves 902 are formed in a manner in which the coupler layer 924, the upper contact layer 923, the second absorber layer 932, the middle contact layer 922, and the first absorber layer 931 are removed. Part of a surface of the lower contact layer 921, the side surfaces of each pixel 901, and part of the upper surface of the coupler layer 924 are covered by passivation films 950.
In each pixel 901, a wiring line 961 connected to the middle contact layer 922 is formed on the corresponding passivation film 950. The lower contact layer 921 forms a first common electrode and is connected to a wiring line 962 formed on the corresponding passivation film 950. The upper contact layer 923 of each pixel 901 is connected to a wiring line 963 formed on the corresponding passivation film 950 with the coupler layer 924 interposed therebetween and forms a second common electrode.
The two-wavelength infrared photodetector is connected to a drive circuit 980 with bumps 971, 972, and 973 interposed therebetween. The wiring line 961 connected to the middle contact layer 922 formed in each pixel 901 is connected to a transistor 981 of the drive circuit 980 with the bump 971 interposed therebetween. The wiring line 962 connected to the lower contact layer 921 is connected to an electric potential VB of the drive circuit 980 with the bump 972 interposed therebetween. The wiring line 963 connected to the upper contact layer 923 with the coupler layer 924 interposed therebetween is connected to an electric potential VA of the drive circuit 980 with the bump 973 interposed therebetween.
The two-wavelength infrared photodetector having the structure illustrated in FIG. 1 may detect infrared of two wavelengths in a manner in which the time in which the first absorber layer 931 operates and time in which the second absorber layer 932 operates are divided. Specifically, an electric potential difference is generated between the lower contact layer 921 and the middle contact layer 922, and the electric potential of the upper contact layer 923 and the electric potential of the middle contact layer 922 are made equal. Thus, carriers that have absorbed infrared of a first wavelength that is incident on the first absorber layer 931 are extracted from the middle contact layer 922. At this time, since the upper contact layer 923 and the middle contact layer 922 have an equal electric potential, no carriers are extracted from the middle contact layer 922 even through infrared of a second wavelength is incident on the second absorber layer 932. In this way, optical signals only through the first absorber layer 931 may be detected.
An electric potential difference is generated between the upper contact layer 923 and the middle contact layer 922, and the electric potential of the lower contact layer 921 and the electric potential of the middle contact layer 922 are made equal. Thus, carriers that have absorbed infrared of the second wavelength that is incident on the second absorber layer 932 are extracted from the middle contact layer 922. At this time, since the lower contact layer 921 and the middle contact layer 922 have an equal electric potential, no carriers are extracted from the middle contact layer 922 even through infrared of the first wavelength is incident on the first absorber layer 931. In this way, optical signals only through the second absorber layer 932 may be detected.
The two-wavelength infrared photodetector having the structure illustrated in FIG. 1 may detect infrared of two different wavelengths in a time division manner, as described above.
In the case where infrared of the first wavelength is detected in the first absorber layer 931, the electric potential of the middle contact layer 922 and the electric potential of the upper contact layer 923 are requested to be equal. The reason is that in the case where the electric potential of the middle contact layer 922 and the electric potential of the upper contact layer 923 are different from each other, a photo current flows due to a difference in electric potential between the middle contact layer 922 and the upper contact layer 923 when infrared of the second wavelength is incident on the second absorber layer 932. Accordingly, in this case, it is difficult to separate the infrared of two different wavelengths. The same is true in the case where infrared of the second wavelength is detected in the second absorber layer 932.
FIG. 2 is a circuit diagram including a pixel of the two-wavelength infrared photodetector and part of the drive circuit 980. The electric potential VS of the middle contact layer 922 is indirectly determined by the gate potential VIG of the transistor 981 of the drive circuit 980 and a drain current flowing through the transistor 981. Accordingly, electric potentials that can be optionally set from the outside include the electric potential VA of the upper contact layer 923 and the electric potential VB of the lower contact layer 921, which are electric potentials of the two common electrodes, and the gate potential VIG of the transistor 981. For this reason, it is difficult to apply an electric potential directly to the middle contact layer 922 for reading signals.
In the case where the pixels 901 are disposed in the two-wavelength infrared photodetector, the characteristics of the first absorber layer 931 and the second absorber layer 932 in each pixel 901 are not the same and are different between the pixels 901. In this case, even when the equal electric potentials VA, VB, and VIG are applied to each pixel 901, the electric potential VS of the middle contact layer 922 varies between the pixels because differences in the characteristics of the first absorber layer 931 and the second absorber layer 932 in each pixel 901 cause the flowing electric current to vary.
In the case of operation as an imaging device, even if the characteristics of all of the pixels are the same, the amount of incident light varies between the pixels, and generated signals vary between the pixels. For this reason, since the electric potential VS of the middle contact layer 922 depends on the magnitude of the electric current, the electric potential VS varies between the pixels.
The electric potential VS of the middle contact layer 922 thus varies between the pixels, a noise current as described above is made, and it is difficult to sufficiently separate infrared of two different wavelengths.
To solve such a problem, there is disclosed a method for separating infrared of two different wavelengths, for example, in a manner in which the middle contact layer is formed of three semiconductor layers of an n-type layer, a p-type layer, and an n-type layer, and flow guiding by using a pn junction is used. There is also disclosed a method for separating infrared of two different wavelengths in a manner in which barrier layers are formed of a material having a large band gap between the middle contact layer and the first absorber layer and between the middle contact layer and the second absorber layer. This method enables signals of two wavelengths to be easily separated in a manner in which the barrier layers that have a large band gap and are not doped with impurity elements are formed such that no electric current flows in the direction opposite to the direction in which the electric current flows through the first absorber layer and the second absorber layer during operation.
According to the above method, however, compound semiconductor layers such as the barrier layers are formed at a relatively high temperature in the case where the compound semiconductor layers are formed by epitaxial growth. For this reason, when the barrier layers are formed, in some cases, impurity elements with which the middle contact layer is doped segregate to the barrier layers, which are not doped with impurity elements, and the electric current flows in the direction opposite to the direction in which the electric current flows during operation. In this case, it is difficult to stably separate signals of two wavelengths.
For these reasons, there is a demand to stably separate signals of two wavelengths in the two-wavelength infrared photodetector in which the absorber layers are stacked.
The followings are reference documents.
[Document 1] Japanese Laid-open Patent Publication No. 2010-192815 and
[Document 2] Japanese Laid-open Patent Publication No. 2015-142110.